The invention relates to a semiconductor light emitting device such as a semiconductor laser. More particularly, the present invention relates to a semiconductor light emitting device which consists of a semiconductor of gallium nitride type compound and is suitable for emitting blue light.
The semiconductor of gallium nitride (GaN) type compound described here is referred to as a semiconductor comprising a GaN compound which consists of Ga of group III element and N of group V element, or III-V compound in which part of Ga of GaN is substituted by other group III element such as Al or In and/or part of N of GaN is substituted by other group V element such as P or As.
In the past, there has been extensively spread the technology for manufacturing a semiconductor laser which emits infrared rays or red rays using GaAs. In contrast, there has been desired a semiconductor laser which emits in a visible ray area blue ray of shorter wave length than that of the foregoing semiconductor laser, by using a semiconductor of GaN type compound, it has become possible to manufacture a light emitting diode (hereinafter referred to as LED) which emits blue light, while the development of a semiconductor laser which emits blue light is under way. In an LED which uses the semiconductor of gallium nitride type compound, during epitaxial growth of a p-type layer, comprising Ga and N, since a p-type dopant Mg or Zn is easily coupled with H atoms in the carrier gas such as H.sub.2 or the reactant gas such as NH.sub.3 and the p-type dopant does not fully fulfill the functions thereof, the grown p-type layer is activated by annealing in order to reduce the resistance by separating the coupling with Mg or Zn and H.
On the other hand, in such a semiconductor laser as this which uses a semiconductor of GaN type compound, it is difficult to manufacture such a semiconductor laser which corresponds to a GaAs type compound semiconductor laser of refractive index guiding structure which contain both the light absorption layer and the current blocking layer. In other words, in order to make the current blocking layer as the absorption layer, the band gap energy of the current blocking layer must be made to be equal or smaller than that of the active layer, therefore, if a semiconductor of InGaN type compound is used for the active layer, a semiconductor material having large composition ratio of In must be used for the current blocking layer. However, when the composition ratio of In increases, the atom of In easily evaporates at the normal growth temperature, and it is difficult to deposit a film of compound semiconductor, while controlling the composition ratio.
Generally, as a wave guide structure of a semiconductor laser, a refractive index guiding structure and a gain guiding structure are known. The refractive index guiding structure is provided with a difference of refractive index in parallel direction with respect to the active layer so as to confine and direct the light, and thus it is possible to obtain a single lateral mode oscillation up to a high output operation, but the coherence possibility is high and the noise induced by the return light (optical feedback noise) is apt to generate. On the other hand, the gain guiding structure is one which does not have a difference of refractive index in lateral direction, in which the lateral mode is unstable and kink is apt to occur, but the optical feedback noise is low because the longitudinal multiple mode is oscillated.
As a structure of a semiconductor laser suitable for the gain guiding structure which uses a semiconductor of GaN type compound, one shown in FIG. 16 or FIG. 17 can be considered. To describe the one shown in FIG. 16, on a sapphire substrate 1 are laminated in order a buffer layer 2 consisting of GaN, a lower cladding layer 3 consisting of Al.sub.z Ga.sub.1-z N (0&lt;z&lt;1), an active layer 4 consisting of In.sub.x Ga.sub.1-x N (0&lt;x&lt;1), an upper cladding layer 5 consisting of Al.sub.z Ga.sub.1-z N, and a contact layer 8, and further an upper electrode 9 in the form of a stripe is provided thereon. Further, part of the layers laminated are removed until part of the lower cladding layer 3 or the buffer layer 2 is exposed, and a lower electrode 10 is provided on the exposed surface. In this case, when voltage is applied between the upper and lower electrodes 9 and 10, electric current flows only to part of the central portion in the active layer 4 according to the shape of the upper electrode 9 so as to make such part into an active area where laser light is generated. However, in a semiconductor laser of such a structure as this, it becomes difficult to control the electric current to be injected into the active area.
With respect to FIG. 17 in which portions corresponding to the same portions shown in FIG. 16 are provided with the same numerals, for the purpose of leaving the upper electrode 9 and the semiconductor layers thereunder in the form of a stripe, the portion of both sides thereof is etched and removed from the top halfway down the upper cladding layer 5, and made into a mesa-type shape. In accordance with such structure as this, the control of the electric current to be injected becomes easy as compared to the structure shown in FIG. 16, but the control of dimensions in manufacturing is difficult, the side wall of the stripe-like portion to be removed and exposed by etching is susceptible to damage by the etching, and a semiconductor laser with good quality cannot be obtained.
On the other hand, an example of a conventional semiconductor laser of the refractive index guiding structure which uses a semiconductor of GaAs type compound is shown in FIG. 18. In FIG. 18, the numeral 21 represents a semiconductor substrate consisting, for example, of an n-type GaAs or the like, on which are laminated in order a lower cladding layer 22 consisting, for example, of an n-type Al.sub.v Ga.sub.1-v As (0.35.ltoreq.v.ltoreq.0.75), an active layer 23 consisting, for example, of a non-doping type or n-type or p-type Al.sub.w Ga.sub.1-w As (0&lt;w&lt;0.7, w&lt;v) a first upper cladding layer 24 consisting of a p-type Al.sub.v Ga.sub.1-v As, a current blocking layer 25 consisting of an n-type GaAs, a second upper cladding layer 26 consisting of a p-type Al.sub.v Ga.sub.1-v As, and a contact layer 27 consisting of a p-type GaAs, on the top surface and the bottom surface are respectively provided a p-side electrode 28 and an n-side electrode 29, and a chip of a semiconductor laser is formed. In this structure, the current blocking layer 25 consisting of the n-type GaAs is a conductive type layer which is different from the p-type cladding layer in the neighborhood, wherein the gap energy of a pn junction is utilized to block electric current, injection current is restricted to the stripe-like active area of width W, and by absorbing the light generated in the active layer (that is, as a light absorption layer), a difference of refractive index is provided inside and outside of the stripe. Therefore, the light is confined in the lateral direction, and is used as a semiconductor laser of refractive index guiding structure of red ray or infrared ray which stably directs the wave in a stripe-like width W of the active layer 23a.
In this structure, by using a material which does not absorb the light as the current blocking layer 25, and by keeping away the current blocking layer 25 from the active layer 23 at the distance, a semiconductor laser of gain guiding structure can be obtained, but in a semiconductor laser which uses a semiconductor of GaN type compound in particular, when GaN is used as the current blocking layer 25 and the distance from the active layer is kept away to obtain the gain guiding structure, leakage current increases, so that a suitable material as the current blocking layer 25 is desired.
Further, an example of a semiconductor laser of the gain guiding structure which uses a semiconductor of a conventional GaAs type compound is shown in FIG. 19. In this structure, a high resistance layer 30 is formed by implanting the protons and the like on both sides of the current injection area, and the current injection area is controlled in the same manner as the construction shown in FIG. 16 through FIG. 17 and is made a gain guiding structure. The portions which are the same as those shown in FIG. 18 are designated with the same symbols and the description thereof is omitted.
The band gap energy and the refractive index of the semiconductor of GaN type compound are different from those of the semiconductor of GaAs type compound respectively, and therefore, it is not possible to obtain a semiconductor laser which uses a semiconductor of GaN type compound having the same structure as that of a semiconductor laser which uses a semiconductor of GaAs type compound. In a semiconductor laser of the structure shown in the foregoing FIG. 16, it is necessary to provide a large distance between the electrode 9 and the active layer 4, and because it is not possible to excessively increase the carrier density of a p-type layer of the semiconductor of GaN type compound and thus the electric current is difficult to flow, power consumption becomes large and the light emitting efficiency is lowered. In the structure shown in FIG. 17, etching must be provided to obtain a mesa-type shape, but it is difficult to etch the semiconductor of GaN type compound as compared with that of GaAs type compound, and when providing the wet etching, the semiconductor must be etched for about 1 to about 60 minutes at high temperature of more than 150 to 250.degree. C., it takes longer time for etching at low temperature, while the control of etching at high temperature is difficult. In the case of dry etching, there is a problem in that reactive ion etching must be performed under the atmosphere of chlorine gas, so the etched surface is damaged and attachment of contamination occurs during etching.
In order to provide a current blocking layer comprised of a material which does not absorb light, there is a problem, as described above, in that the leakage of light is large even if GaN is used and the light emitting efficiency is lowered, and the material suitable for the current blocking layer is not obtained.
In order to provide a current blocking layer comprised of a material capable of absorbing light (light absorption material), there is a problem, as described above, in that it is difficult to deposit a film of light absorption material consisted of a semiconductor of GaN type compound, and a semiconductor laser of complex refractive index guiding structure cannot be obtained.
On the other hand, in order to obtain a semiconductor laser of stable oscillation with low optical feedback noise and high kink level, it is preferable to employ the strong point of both types of the foregoing refractive index guiding structure and the gain guiding structure, but the current blocking layer made from a suitable material capable of absorbing light has not been obtained.
Further, as described above, in a semiconductor laser of the structure shown in FIG. 18, when much light of the active layer 23 is caused to be absorbed by the current blocking layer 25, such structure will be a refractive index guiding structure which is apt to perform single longitudinal mode oscillation with high coherence possibility. Therefore, the optical feedback noise is easily generated. On the other hand, when the light of the active layer is not caused to be absorbed by the current blocking layer, a gain guiding type structure which does not have a difference of refractive index in lateral direction is obtained, but in the semiconductor laser of gain guiding structure, the optical feedback noise becomes small while the astigmatic difference becomes large, and further the lateral mode is not stable, and kink is apt to occur easily. The nature of both the refractive index guiding structure and the gain guiding structure differ delicately depending not only on the material of the current blocking layer but also on the conditions such as the distance between the active layer 23 and the current blocking layer 25, that is, the thickness of the first upper cladding layer, and the width of the stripe-like current injection area formed in the current blocking layer 25, so that there occurs a complicated correlation in conjunction with the absorption characteristic involved in the material of the current blocking layer 25. So there is a problem in that a semiconductor laser restricting noise generation and having the desired high characteristic cannot be obtained.
In a conventional semiconductor laser of electrode stripe-like structure, there is a problem in that when the distance between the active layer and the electrode increases, the leakage current which diffuses in outer of the width of the electrode stripe and does not contribute to the emission of light increases, and especially because the operating voltage is about 3 V in a semiconductor laser of gallium nitride type compound, if the leakage current increases, the waste of electric power which does not contribute to the emission of light increases and it causes the light emitting efficiency to lower.
In accordance with a method whereby the proton or the like is implanted into the non-current injection area to provide high resistance on both sides of the electrode stripe, there is a problem in that an ion implanter will become a large-scale apparatus and it is not suitable for mass production in which a low cost is required.
Further, in accordance with a method whereby part of semiconductor layers are etched and made into a mesa-type shape, there is a problem in that it takes time for etching and especially it is difficult to etch semiconductor layers of gallium nitride type compound, either the wet etching must be performed at high temperature of more than 250.degree. C. or the reactive ion etching must be performed under the atmosphere of chlorine gas, involving hard works.
In each of the foregoing conventional semiconductor laser, all the both end faces where the stripe-like active area 23a is exposed are vertical with respect to the direction of laminating these semiconductor layers, and optical resonator is formed between the both end faces but the end faces absorb the light, and in turn deterioration due to temperature rise cannot be avoided in such a structure.
Further, because a cleavage is performed in manufacturing, so the length of the resonator is restricted, that is, there has been manufactured only a semiconductor laser having resonator of more than 200 .mu.m. From the standpoint of the required intensity of the laser ray to be generated, it is possible to manufacture a semiconductor laser even if the length of a resonator is less than 100 .mu.m, but a semiconductor laser having such a resonator has not been manufactured yet.
In addition, in such a semiconductor light emitting device as a conventional semiconductor laser or an LED wherein the light emitting layer (active layer) is provided between the cladding layers, and the light is emitted from the end face in parallel to semiconductor layers laminated. So if it is required to emit the light in a vertical direction to a surface of a semiconductor chip (semiconductor layers laminated) which is mounted on a submount or a lead frame, it is necessary to provide newly a reflection surface 20, as shown in FIG. 20 for example, on the outside of the end face of a semiconductor layer 11 so as to direct a light path 13 into vertical direction to a surface of a semiconductor chip.